Evaluation Of Ultra-deep Fractured Tight Sandstone Gas Reservoir Damage

Tight sandstone gas reservoir has abundant hydrocarbon resources with great developmental potential. Tight sandstone gas reservoir at the J area has characteristics of extremely deep buried, high temperature, high pressure, fracture developed and matrix tightness, which lead to complex damage mechanism and hard to evaluate formation damage during the drilling, completion and exploitation stages. Therefore, it is necessary and emergent to evaluate this kind of formation damage effectively by clarifying its damage mechanism, so as to optimize the working fluids for protecting gas reservoir.Taking tight sandstone gas reservoir in J area of Ta Li Mu basin as research object, by analyzing its temperature, pressure, sensitive minerals, pore and throat structure, crack characteristic and engineering features, this study pointed out potential damage factors and explored an appropriate method to evaluate the damage of extra deep buried and fracture developed tight sandstone gas reservoir. Damage mechanism was revealed by damage evaluation research on matrix and fracture core. The main achievements and conclusions are as follows:Pinpointed the potential damage factors of extremely deep buried and fracture developed tight sandstone gas reservoir. The average porosity of reservoir is ?3% and the average permeability is ?0.01mD. Major pore type is intergranular dissolved pore, having the flaky and curved lamellar throat with large tortuosity. Main pore throat radius is nanoscale. Clay minerals content is between 1.73% and 4.95%. Illite is the major part and also contain a small amount of illite/smectite interstratified mineral, kaolinite and chlorite. Illite is flake, fibrous and bridge like, which exists on the intergranular pore and surface of microcracks. Fractures are mainly of high angle or vertical cracks, none to half filling. In situ effective width is from 50 to 300 microns and leakage is obvious during drilling and completion operations.Proposed a high temperature and high return pressure liquid measure method, which was applied to evaluate fluids sensitivity of tight sandstone. Adding high return pressure at the core outlet face could increase feeding efficiency of tight sandstone core, shorten measuring time and enhance measuring accuracy. Evaluated compatibility of sensitive minerals with external fluids and confirmed both steady state method and unsteady state method being able to evaluate fluids sensitivity of tight sandstone.Revealed extremely deep buried and high temperature tight sandstone gas reservoir sensitivity damage degree and mechanism. The gas layer damage degree ranking:matrix alkali sensititvity> crack alkali sensitivity> crack water sensitivity> matrix water sensitivity, the damage rate is between 41% and 91%. The main damage mechanisms are:sensitive minerals are not compatible with extraneous fluids. Hair-like and silk-like illite are dispersed inducing particles migration, hydration and swelling of illite/smectite interstratified mineral, plugging nanoscale pore and throat in tight sandstone core; infiltrating flow channel in matrix core is nanoscale. Flaky and curved lamellar throat is major proportion,which is easy to close under high effective stress leading to matrix core moderately strong/strong stress sensitivity damage. As strong particle supporting effect on the fracture plane, crack core shows weak or moderately weak stress sensitivity damage.Evaluated liquid phase trapping damage of extremely deep buried and fissured tight sandstone gas reservoir. Water and oil contact angle on tight sandstone surface are 68° and 24° respectively, presenting indermediate wetting characteristics. Major matrix pore throat is nanoscale, inducing liquid phase imbibition saturation reaching about 78%, after flowing back still as high as 62%, showing easy to invade but difficult to flowback, as a result inducing liquid phase trapping damage. The damage rate is between 67% and 84%, existence of crack exacerbates liquid phase trapping damage in matrix pore throat.Carried out drilling and completion fluids damage evaluation of extremely deep buried tight sandstone gas reservoir and proposed some countermeasures to protect formation during drilling and completion. Drilling and completion fluids inducing permeability damage degree is severer than drilling and completion filtrate. Solid particles blocking pore throat and microcracks is principal reason. Utilized finite element method to evaluate crack width response to dynamic pressure variation of well bore. This paper provides a theoretical basis for optimal selection of drilling fluids, completion fluids and temporary plugging particles to effectively block the cracks in deep buried tight sandstone gas reservoir.